This invention relates to the field of optical fibre telecommunications, and more particularly to telecommunications over long distances. The invention is particularly concerned with optical communication systems using return-to-zero (R-Z) pulses, and the problems associated with distortion in the pulse shapes over long distances.
It is known that a xe2x80x9csolitonxe2x80x9d type signal can be propagated along a dispersive fibre without significant chromatic dispersion, because the dependence of the refractive index on the intensity of the signal is able to oppose chromatic dispersion. Thus, the spectral form of the signal is preserved over distance.
There are, however, numerous other distortions which arise during the transmission of optical pulses. These can lead to timing jitter or amplitude noise.
It has been recognised that it is desirable to constrain timing jitter and amplitude noise without the need for full signal regeneration so that the distance between regenerators can be increased. One known approach for improving timing control involves the use of a synchronous modulator which truncates the rise and fall edges of the optical pulses. Appropriate control of a modulator in this way can also reduce amplitude noise at the same time. For example, a pulse having increased amplitude as a result of amplitude noise will, after passage through the timing control modulator, have larger bandwidth than pulses of lower amplitude. The fibre non-linearity gives rise to increased filtering of higher bandwidth signals, thereby bringing down the amplitude. Thus, it has been appreciated that the use of timing control may combine with the non-linear characteristics of the optical fibre in such a way that the optical pulses tend towards a uniform amplitude. Optical filtering elements may also be used to achieve the desired filter characteristics.
A problem with this approach is that it places constraints upon the fibre dispersion characteristics, the span lengths and the power budgets for the signals. All of these considerations need to be taken into account to enable a system to be designed which achieves the desired control of the pulse shape. This approach is not suitable for low power systems, such as WDM optical transmission systems.
According to the present invention there is provided an apparatus for reshaping optical pulses comprising:
an input for receiving an optical pulse train;
a divider for dividing the incoming optical signal into a first and a second path;
a converter in the second path for converting the signal into an electrical signal, the electrical signal providing information concerning the pulse power of individual pulses;
an optical modulator provided in the first path for modifying the optical signal in the first path, the modulator being controlled by a signal derived from the electrical signal so as to attenuate the pulses by an amount which depends on the power of the pulses, higher power pulses being attenuated more than lower power pulses.
In the arrangement of the invention, higher power pulses are attenuated more than lower power pulses, which tends to even out pulse powers to a constant level. This helps maintain a constant signal to noise ratio within the system. This can extend the number of optical spans of an optical signal before it requires full electrical regeneration. The reshaping apparatus is effectively implementing a non-linear transfer function which attenuates optical pulses according to their power.
The converter preferably comprises a photodetector for detecting the individual pulses, and circuitry for providing a signal for controlling the modulator depending on the pulse powers.
The modulator control signal may be correctly timed by so-called dead reckoning, by which a delay element is provided either in the optical path and/or in the electrical path, selected to that the delays in the two paths are equal. This ensures that the modulator is being controlled at the correct time corresponding to the arrival of the optical pulse.
The apparatus may further comprise a clock extraction circuit for extracting a clock signal from the electrical signal, and wherein the modulator is further controlled to truncate the leading edges of early pulses and to truncate the trailing edges of late pulses. In this way, the clock extraction enables jitter control to be implemented. For a given transmission network, the reshaping apparatus of the invention can then reduce the number of regenerators required.
The optical modulator may comprise a first modulator for amplitude reshaping and a second modulator for timing reshaping, or else a single modulator may be controlled to perform both functions.
The reshaping apparatus of the invention can also improve the tolerance of the optical transmission system to optical distortion effects, such as polarisation mode dispersion and other non-linear effects.
Preferably, the clock extraction unit has a narrower bandwidth than that required for detection of the data.
The invention also provides an optical network comprising optical fibre connections between end stations, the optical fibre connections comprising spans between regenerators, at least one apparatus of the invention being provided in one or more spans. The introduction of the apparatus of the invention into the fibre network can increase the total transmission distance of the system or can reduce the number of regenerators required.
The invention also provides a method of reshaping optical pulses comprising:
dividing an input optical pulse train into a first and a second path;
converting the signal in the second path into an electrical signal which provides information concerning the pulse power of individual pulses;
controlling an optical amplitude modulator provided in the first path using a signal derived from the electrical signal so as to attenuate the pulses by: an amount which depends on the power of the pulses, higher power pulses being attenuated more than lower power pulses.
Preferably, the method also involves extracting a clock signal from an input optical pulse train, and using the clock signal to reshape the leading and trailing edges of the pulses.